I changed to using a map as suggested and I'm not getting an error when updating using dataloader, but my invoice totals are incorrrect.  However, if I just update one record the invoice totals are correct.  Not sure why the totals are off when run in batch.

trigger SalesInvoiceTotalsToOpportunity on Sales_Invoice__c (after insert, after update) {

	Set<Id> opIds = new Set<Id>();
	for (Sales_Invoice__c si : Trigger.new) {
		opIds.add(si.Opportunity__c);
	}
	
	List<Sales_Invoice__c> salesInvoiceList = [Select Opportunity__c, Date__c, Payment_Posting_Date__c, Subtotal__c, Amount_Paid__c From Sales_Invoice__c Where Opportunity__c in: opIds];
	
	Map<Id, List<Sales_Invoice__c>> oppyToSalesInvoiceMap = new Map<Id, List<Sales_Invoice__c>>();
	
	for (Sales_Invoice__c salesInvoice : salesInvoiceList) {
		
		List<Sales_Invoice__c> salesInvoices = oppyToSalesInvoiceMap.get(salesInvoice.Opportunity​__c);
		if (salesInvoices == null) { 
			
			salesInvoices = new List<Sales_Invoice__c>();
			oppyToSalesInvoiceMap.put(salesInvoice.Opportunity​__c, salesInvoices);
		}
		salesInvoices.add(salesInvoice);
	}
	
	List<Opportunity> opps = [Select Date_Paid__c, Date_Invoiced__c, Total_Invoiced_Amt__c, Total_Paid_Amt__c From Opportunity Where Id in: opIds];

	Map<Id, Opportunity> oppsToUpdate = new List<Id, Opportunity>();
	
	for (Id oppyId : oppyToSalesInvoiceMap.keySet()) {
		Decimal invoicedAmt = 0;
		Decimal totalPaid = 0;
		List<Sales_Invoice__c> salesInvoices = oppyToSalesInvoiceMap.get(oppyId);
		for (Sales_Invoice__c salesInvoice : salesInvoices) {
			invoicedAmt =  invoicedAmt + salesInvoice.Subtotal__c;
			totalPaid = totalPaid + salesInvoice.Amount_Paid__c;
		}
		
		for (Opportunity o : opps) {
		o.Total_Invoiced_Amt__c = invoicedAmt;
		o.Total_Paid_Amt__c = totalPaid;
		oppsToUpdate.put(o.Id, o);
		}
	}
	
	
	update oppsToUpdate.values();

}

I'm totally all the invoices for a specific opportunity.  So, if there are three invoices for Opportunity A, I add the totals for the 3 invoices and update the total into the opportunity.  Again, this works for a single opportunity, but when I am running in batch for multiple opportunities, the summations are not correct.  Do you see where the error is occurring in my code?

Thanks.